1. Field of the Invention
The present invention relates to a driving apparatus and a method of generating a driving signal, and more particularly to a driving apparatus and a method of generating a driving signal capable of reducing power consumption.
2. Description of Related Art
One of the characteristics of liquid crystal molecules is that they cannot remain unchanged under a certain fixed voltage. Otherwise, as time goes on, even if no voltage is applied, the liquid crystal molecules may not be able to rotate in response to an electric field variation as a result of characteristic disruption of the liquid crystal molecules so that different gray scales are formed. Therefore, driving the liquid crystal molecules by time varying applied voltage prevents characteristic disruption of the liquid crystal molecules is prevented. For example, a positive polarity voltage is applied to the liquid crystal molecules to rotate the liquid crystal molecules in a positive direction and then a negative polarity voltage is applied to the liquid crystal molecules next to rotate the liquid crystal molecules in a negative direction.
There are two methods of driving the liquid crystal molecules, one is a fixed common voltage driving method (normally labeled Vcom) while the other is a variable common voltage driving method. FIG. 1 is a diagram showing a driving method having a fixed common voltage. As shown in FIG. 1, this driving method uses the common voltage as a center voltage such that the output voltage of an operational amplifier (OPAMP) is divided into a positive and a negative polarity. Furthermore, each gray scale data (for example, 00˜FF) has its voltages corresponding to a positive and a negative polarity. The output when the output voltage of the operational amplifier is greater than the common voltage is regarded as a positive polarity output and the output when the output voltage of the operational amplifier is smaller than the common voltage is regarded as a negative polarity output.
Regardless of whether the output voltage of the operational amplifier is a positive polarity or a negative polarity, in other words, regardless of whether the output voltage of the operational amplifier is higher or the common voltage is higher, a group of gray scales with identical brightness is produced as long as a fixed voltage difference exists between the output voltage of the operational amplifier and the common voltage. Although the gray scales are expressed identically, the difference in the direction of rotation of the liquid crystal molecules with respect to the positive and the negative polarity prevents the direction of rotation fixed in the same direction and leads to characteristic disruption of the liquid crystal molecules.
However, the foregoing driving method has its defects. For example, assuming a complete black frame (gray scale data is 00) needs to output on a display panel. When the output voltage of the operational amplifier is a positive polarity and if the gray scale data 00 is smaller than G1 (G1 and G2 are any random gray scale data), then the output voltage of the operational amplifier must reach the level in region 1. On the other hand, when the output voltage of the operational amplifier is a negative polarity and if the gray scale data is smaller than G2, then the output voltage of the operational amplifier must reach the level in region 4. More simply, the output voltage of the operational amplifier vary over a large voltage range when the liquid crystal molecules make a transition from a positive polarity to a negative polarity.
Similarly, the driving method having a variable common voltage also has the same problem. FIG. 2 is a diagram showing a driving method having a variable common voltage. As shown in FIG. 2, each gray scale data (for example, 00˜FF) still has its voltages corresponding a positive and a negative polarity. The output when the output voltage of the operational amplifier is greater than the common voltage is regarded as a positive polarity output and the output when the output voltage of the operational amplifier is smaller than the common voltage is regarded as a negative polarity output. Regardless of whether the output voltage of the operational amplifier is higher or the common voltage is higher, a group of gray scales with identical brightness is produced as long as a fixed voltage difference exists between the output voltage of the operational amplifier and the common voltage.
However, the output voltage of the operational amplifier changes over a wide range under the following four conditions. First, when the variation in the gray scale data is of the same polarity, assuming a positive polarity, and inter-converts between 00 and FF, if the gray scale data is smaller than G1 (G1, G2, G3 and G4 are random gray scale data), and the gray scale data FF is greater than G4, then the output voltage of the operational amplifier transits between region 1 and region 4. Second, assuming the variation of the gray scale data is a transition between the negative polarity gray scale data 00 and the negative polarity gray scale data FF, and the gray scale data FF is greater than G3, then the output voltage of the operational amplifier transits between region 3 and region 2.
Third, when the variation of the gray scale data has different polarities, assuming the gray scale data is 00, and the polarity transits between the positive polarity and the negative polarity, if the positive polarity gray scale data 00 is smaller than G1 and the negative polarity gray scale data 00 is smaller than G2, then the output voltage of the operational amplifier transits between region 1 and region 3. Fourth, assuming the variation of the gray scale data transits between the positive polarity gray scale data FF and the negative polarity gray scale data FF, if the positive polarity gray scale data FF is greater than G3 and the negative polarity gray scale data FF is greater than G4, then the output voltage of the operational amplifier transits between region 2 and region 4.
As mentioned in the foregoing description, when the output voltage of the operational amplifier varies over a large range so that the transition time of the output voltage of the operational amplifier is longer, the stabilization time of the voltage change is also longer. Hence, the application range is restricted. From the perspective of slew rate (SR, that is, the largest output voltage variation permitted of an electronic device dV0(t)/dt), the slew rate of an operational amplifier may be represented by using a formula SR=I/Cc, wherein I is the current of the operational amplifier, Cc is a compensation capacitance coupled to the operational amplifier. If the output voltage difference of the operational amplifier is 10 volts (V) and the transition is completed in 5 μs, then the slew rate SR is 2V/μs. However, if the transition is completed in 2.5 μs, then the slew rate SR is 4V/μs. In other words, the current of the operational amplifier has to be increased one fold or the compensation capacitance has to be reduced by half.
The compensation capacitance may be reduced so as to shorten the stabilization time of the output voltage transition of the operational amplifier. In general, the smaller the compensation capacitance, the shorter will be the transition time of the output voltage of the operational amplifier because the transition time of the output voltage of the operational amplifier is proportional to the compensation capacitance. However, this will easily lead to oscillation in the operational amplifier. Alternatively, the current of the operational amplifier may be increased. In general, the greater the current of the operational amplifier, the shorter will be the transition time of the output voltage of the operational amplifier because the transition time of the output voltage of the operational amplifier is inversely proportional to the current of the operational amplifier. Yet, quickening the stabilization time by purely increasing the current of the operational amplifier will lead to a significant increase in power consumption that might adversely affect product competitiveness.